Hydraulic control systems are often used to control the position of an aircraft flight control surface. A typical hydraulic control system may include a modulating piston, a servo valve and an actuator. The modulating piston mechanically controls the position of the slide of the servo valve and the servo valve controls the flow of fluid to and from the actuator. The actuator positions the flight control surface.
If the hydraulic control system malfunctions, the pilot may lose control of the aircraft, and a crash may result. It is very important, therefore, that the hydraulic control system be very reliable.
One failure that can occur in hydraulic system of this type is the sticking or jamming of the movable elements of the modulating piston or the servo valve. Various techniques are known for preventing a catastrophic failure if one of the slides of the servo valve jams. Servo valves having some form of anti-jam protection are shown, by way of example, in Mott U.S. Pat. No. 2,613,650, Richolt U.S. Pat. No. 3,253,613, and York et al., U.S. Pat. No. 3,439,707.
Jam protection for modulating pistons and similar devices which drive the servo valve has apparently been given much less attention. A typical modulating piston may include, for example, a housing, a piston slidable in the housing, and a drive rod coupled to the piston for mechanically stroking the slide of the servo valve. The position of the piston within the housing may be controlled, for example, by fluid under pressure ported to the opposite faces of the piston by a suitable device, such as an electrohydraulic valve. The electrohydraulic valve may in turn be controlled directly by the pilot of the aircraft or by an automatic mechanism, such as an auto-pilot.
One prior art device which provides some jam protection for modulating pistons includes two modulating pistons of the type described hereinabove with the drive rods from the two modulating pistons being coupled by a spring detent unit. This does provide some jam protection. However, the springs provide very undesirable hysteresis and, when a jam occurs, the output stroke from the modulating piston combination is reduced by 50 percent. In addition, if a jam occurs, it is necessary to overcome the spring force from the spring detent unit. Thus, this prior art device is less than satisfactory.